1. Field of the Invention
This invention relates to a process for bonding steel sheets, such as chromated steel sheets, tin plates, or untreated rolled steel sheets, with a polyamide adhesive which comprises applying first a primer of 0.1 to 2.5 weight parts of a bisphenol A type epoxy resin having a mean molecular weight of more than about 800 and 1 weight part of a thermosetting phenolic resin, which is obtained by reacting a mixture of orthocresol, about 10 to 60 weight parts, and phenol, about 40 to 90 weight parts, with formaldehyde in the presence of an alkali catalyst, on the surfaces of the steel sheets, and then heating the treated steel sheets.
2. Description of the Prior Art
Although many attempts have been made to bond steel sheets with adhesives, fully satisfactory results have not yet been obtained with respect to bonding speed, bonding strength (variations in the bonding strength occur), water proofing properties and heat resistance.
The speed for producing metallic cans by soldering is about 400 cans/min, while that using bonding must be 600 to 800 cans/min. Such high speed bonding work requires a heating and compressing time of at most 10 seconds, preferably 2 to 3 seconds.
Polyamide forms an advantageous adhesive together with an epoxy resin, a phenol resin or a like thermosetting resin, but this adhesive mixture requires a long time for three-dimensional hardening.
For example, the time required for fully hardening the adhesive mixture is several minutes to several tens of minutes at a temperature of 150.degree. to 250.degree.C. Accordingly, the polyamide-thermosetting resin mixture adhesive is not suitable for quick bonding work.
A very high bonding strength must be retained for food cans over a wide temperature range, for example, such cans are subjected to a vapor sterilizing treatment and cold storage at a temperature of -20.degree. to 30.degree.C.
Further, U.S. Pat. No. 3,663,354 discloses a primer which is adhesive to both metals and linear polyamides which is a precondensate of (1) a resole type phenolic resin prepared from a mixture of phenols comprising p-cresol and at least one tri- or higher functional phenol and (2) an epoxy resin. However, the present inventors have unexpectedly found that the present invention can provide a superior bonding strength under a high temperature than that of the above cited U.S. Patent.